Hot-wire anemometers are most often used to measure fluid velocity based on the amount of heat connected away by a fluid passing over a heated wire. In typical hot-wire anemometers, a hot wire or filament is heated by either a constant current (constant-current anemometers) or, alternatively, heated to a constant temperature (constant-temperature anemometers). In either case, the amount of heat lost due to convection is a function of the fluid velocity passing over the filament.
The amount of heat that is dissipated by a heated filament located in a fluid stream depends on a number of factors including the filament's temperature, the geometry of the filament, the temperature of the fluid, and the fluid velocity. The filament's temperature is determined by measuring its electrical resistance. Empirical data and/or mathematical algorithms are used to calculate the temperature and the flow rate based on the measured resistance. Because metals used to fabricate suitable filaments have resistivity coefficients on the order of 0.1%/° C., a high degree of accuracy is needed for measuring the actual resistance of the filament.
One medically-related application for hot-wire anemometers is their use in measuring the inspiration and expiration flow rates of a patient. Many lung function tests require knowing details on the rate at which air is entering and exiting a patient's lungs. Because the maximum realistic flow rate range encountered during inspiration and expiration is relatively low (e.g., between 0 and about 20 L/s), the resistance change in the filament is also small. For example, a filament having a resistance of 2.2 ohms at room temperature may only see a 0.03 ohm change in resistance over the entire realistic flow rate range. Because there is such a small change in the resistance in the filament, it is imperative that this change be measured with great accuracy and precision.
In prior art hot-wire anemometers, for example, as shown in FIGS. 1(a) and 1(b), a single wire anemometer probe 2 is used that is disposed inside a tubular housing. A filament 6 (shown in FIG. 1(b)) is welded between two pins 8 (one is obstructed from view in FIG. 1(a)) that extend from the middle of the probe 2 to outside the housing 4. The probe 2 is detachably attached to a cable 10 which has mating receptacles (not shown) for receiving the two pins 8. The cable 10 communicates with circuitry for calculation of the gas flow rate passing over the filament 6. There are several problems, however, with the prior art probe 2 that prevents the acquisition of accurate and precise resistance measurements.
In the prior art probe 2, there is no way to differentiate between resistivity of the filament 6 and resistivity caused by the cable 10 and the connection between the cable 10 and the two pins 8. Any resistance change caused by the cable 10 and/or the connection will be seen by the circuitry as a change in the resistance of the filament 6, thereby resulting in an erroneous temperature and gas flow calculation. There are several mechanisms by which resistance errors can be introduced in the prior art probe 2. These include, for example, (1) changes in ambient temperature, (2) time variations, and (3) physical disturbance/movement of the cable 10. Some of these errors cannot be eliminated nor reversed without a complete recalibration of the probe 2, which can take a considerable amount of time and effort.
Practical considerations require that the probe be designed in such a manner that allows a user to attach and remove the probe from a cable connecting the probe to the unit housing the electronics. This is particularly true when the probe is disposable or requires frequent replacement, maintenance, or cleaning. Consequently, cables and connectors are virtually required in all probe designs, thereby insuring the existence of the aforementioned error mechanisms.
Thus, there is a need for a detachable hot-wire anemometer probe that can precisely and accurately measure the resistance of a filament without the introduction of resistance errors caused by various environmental artifacts. The probe unit would be modular in that it could be attached/disconnected to a separate device containing the circuitry using a conventional cable.